Abstract/Summary

Space weather events with their solar origin and their distribution through the heliosphere affect the
whole magnetosphere-ionosphere-Earth system. Their real-time monitoring and forecasting are important
for science and technology. Here we discuss one of the largest space weather events of Solar Cycle 23,
in November 2004, which was also one of the most difficult periods to forecast. Nine halo coronal mass
ejections (CMEs), interacting on their way through the interplanetary medium and forming two
geoeffective interplanetary structures, exemplify the complexity of the event. Real-time and
quasi-real-time observations of the ground geomagnetic field show rapid and extensive expansion of the
auroral oval to 55 in geomagnetic latitude accompanied by great variability of the ionosphere.
Geomagnetically induced currents (GICs) seen in ground networks, such as power grids and pipelines,
were significant during the event, although no problems were reported. Forecasts of the CME propagation,
global and local ground geomagnetic activity, and ionospheric parameters, issued by several regional
warning centers, revealed certain deficiencies in predictions of the interplanetary characteristics of the
CME, size of the geomagnetic disturbances, and complexity of the ionospheric variations produced by this
event. This paper is a collective report based on the materials presented at the splinter session on
November 2004 events during the first European Space Weather Week.